Make Counterfactual syntax closer to mathematical notation

[djinnome]

Currently,@ is used to represent an intervention in a counterfactual variable, such as:

Y @ X

Recommendation is to convert this to:

Y(x)

where lower case variables (by convention) represent intervention variables.

Also consider renaming InteventionVariable to Value or CounterfactualValue.

Also, to represent the value of a counterfactual variable, we currently use this notation:

+Y @ -X

Recommend this update:

Y(-x) == y

So now, probabilties can be a dictionary of counterfactual variables and values:

Prob = {Y @ x : -y, Z @ d : z, D : -d}
Prob[Y@x]

This could be represented as a constructor like this:

Pr[Y(x)== y, Z(d) == z, D == -d]